Photoconductive target having arsenicselenium layers of different densities on cryolite layer



y 2, 1968 v. .1. SANTILLI 3,391,297

PHOTOCONDUCTIVE TARGET HAVING ARSENIC-SELENIUM LAYERS OF DIFFERENTDENSITIES ON CRYOLITE LAYER Filed March 29, 1965 2 Sheets-Sheet 1 OUTPUTINVENTOR Vincent J. Suntilli ATTORNE WITNESSES v. J. SANTILLI 3,391,297PHOTOCONDUCTIVE TARGET HAVING ARSENIC-SELENIUM LAYERS OF July 2, 1968DIFFERENT DENSITIES ON CRYOLITE LAYER Filed March 29. 1965 2Sheets-Sheet z L|..I|||| w 5 22% TV FIELDS FIG.4.

TV FIELDS FIG-6- llllllllll 3 2 I m Ewmmau H 2205 TARGET VOLTAGE VOLTSTIME-MINUTES United States Patent 3,391,297 PHOTOCONDUCTIVE TARGETHAVING ARSENIC- SELENIUM LAYERS OF DIFFERENT DENSITIES 0N CRYOLITE LAYERVincent J. Santilli, Corning, N.Y., assignor to Westinghouse ElectricCorporation, Pittsburgh, Pa., :1 corporation of Pennsylvania Filed Mar.29, 1965, Ser. No. 443,566 2 Claims. (Cl. 313-96) ABSTRACT OF THEDISCLOSURE This invention is directed to a radiation pickup tube. Thetube includes a radiation sensitive target which comprises an electricalconductive coating, a layer of cryolite, and a first and second coatingof photoconductive material of different densities.

This invention is directed to an electron discharge device and moreparticularly to an improved radiation sensitive target electrode.

One particular application of this invention is in a photoconductivitytype of pickup tube. The most common type of photoconductivity pickuptube is a vidicon. The vidicon is comprised of an evacuated envelope inwhich there is provided an input radiation transmissive faceplateportion. An electrically conductive coating of input radiationtransmissive material is provided on the inner surface of the faceplate.The conductive layer may be referred to as the backplate or the signalplate of the vidicon. A layer of photoconductivity material sensitive tothe input radiation is deposited on the backplate and the two layers maybe referred to as the target electrode. An electron gun is provided atthe opposite end of the envelope with respect to the faceplate forproviding an electron beam for scanning the tar-get electrode. Suitablescanning means are provided for scanning the electron beam over thetarget member. The beam of electrons for scanning may be of high energytype, that is, of an energy between the first and second crossoverpotential of the target surface or it may be of the more common type oflow energy in which operation is below the first crossover potential ofthe target surface.

In the latter type of operation, the electrons are substantially sloweddown as they approach the target surface and are deposited upon theexposed photoconductive surface to drive the surface down tosubstantially the potential of the cathode of the electron gun. Theconductive signal plate of the target electrode is normally held at apotential of several volts to 100 volts) with respect to the cathode ofthe electron gun. The exposed surface of the target electrode isnormally maintained at the cathode potential prior to excitation by theinput radiation. In this manner, an electric field is provided acrossthe photoconductive layer. When the input radiation image is directedonto the photoconductive layer, the film of photoconductive material isexcited. The excitation of the photoconductive material by the photonscauses a generation of charge carriers, electrons and/ or holes. As aresult of the field impressed across the photoconductive layer, acurrent flow will take place through the layer in the illuminated areasand will cause these areas on the exposed surface of the photoconductivelayer to tend to charge toward the potential of the conductivebackplate. Those areas of the photoconductive layer which are notilluminated will remain at substantially cathode potential. The electronbeam upon scanning over the exposed surface of the photoconductive layerwill return the illuminated target areas to cathode potential. Since thebackplate or signal plate 3,391,297 Patented July 2, 1968 iscapacitively coupled with the exposed surface of the target, theinstantaneous charging of the target by the beam to cathode potentialwill be evidenced by a voltage change in an output circuit electricallyconnected to the conductive backplate. This voltage change is the outputsignal of the vidicon. In those areas that are not illuminated, therewill not be a change in potential and there will not be an output signalderived from these unilluminated areas. The term radiation means notonly electromagnetic radiation such as light but also particlebombardment such as electrons.

'Ihe vidicon type of pickup tube is a simple, reliable and ruggeddevice. This type of tube is very desirable for use in criticalenvironments such as space vehicles. The vidicon tubes of the prior arthave several limitations at least in obtaining a tube having all of thedesirable properties. One of these is lack of good sensitivity. Anotheris a suflicient dark resistivity to give adequate storage Within theoperational environment. Another disadvantage of the prior art type ofdevice is the long response times of the target material.

It is accordingly an object of this invention to provide an improvedradiation sensitive device.

It is a further object to provide an improved radiation sensitive targethaving short integration time.

It is another object to provide an improved target having a shortresponse time.

It is still another object to provide an improved radiation sensitivetarget for high temperature environment.

In accordance with my invention, I provide an improved radiationsensitive target electrode comprised of an electrically conductive layerhaving a layer of insulating material provided on one surface thereofand a radiation sensitive body consisting of two layers, one of a porousmaterial and one of a non-porous material.

Further objects and advantages of the invention will become apparent asthe following description proceeds. The features of novelty whichcharacterize the invention will be pointed out in particularity in theclaims annexed to and forming a part of the description.

For a better understanding of the invention reference may be had to theaccompanying drawings, in which:

FIGURE 1 is a view in section of a pickup tube embodying the teachingsof this invention;

FIG. 2 is an enlarged sectional view of the target shown in FIG. 1;

FIG. 3 is a graphical representation of the short decay of the signalversus time of the target shown in FIG. 1;

FIG. 4 is a graphical representation of short build up time of thesignal versus time of the target shown in FIG. 1;

FIG. 5 is a graphical representation of the sensitivity of the target inFIG. 1; and

FIG. 6 is a graphical representation of the low dark current of thetarget in FIG. 1 in comparison with a prior art device.

Referring now to FIGS. 1 and 2, a pickup tube is illustrated includingan evacuated envelope 12 containing an electron gun assembly 20 and atarget assembly 30. The electron gun 20 consists of at least a cathode22, a control grid 24 and at least one or more accelerating anodes 26and 28 connected by suitable lead-ins to appropriate sources ofpotential for generating and forming an electron beam. The specificdesign of the electron gun 20 is conventional and may be of any suitabletype of electron gun for generating a pencil-like electron beam. Theenvelope 12 includes a faceplate portion 14 of the material such asglass transmissive to the input radiations from a scene. An inputradiation transmissive electrical- 1y conductive coating or film 32 isprovided on the inner surface of the faceplate 14. An insulating coating34 is provided on the conductive layer 32 and two photoconductive layers36 and 37 are provided on the insulating coating 34. The conductive filmor coating 32 is a signal electrode or backplate of the target 30. Anelectrical lead-in 38 is provided to the exterior of the envelope 12.The lead-in 38 is connected through a resistor 41 to a voltage source39. A signal output from the tube is derived across the output resistor41. The target 30 will be described in more detail as to structure andmanufacture in connection with FIG. 2.

Means are provided for focusing the electron beam generated by theelectron gun 20 and scanning the beam over the target 30 to form araster in a conventional well-known means. This may include a focus coil40, deflection yoke 43 and an alignment coil 42. It is also obvious thatelectrostatic deflection and focusing could be utilized in place of theabove-mentioned electromagnetic assembly. An electrically conductivescreen electrode or mesh 33 is positioned adjacent the target 30 andduring operation together with the focus coil 40, functions to insurethat the electron beam from the gun 20 is directed onto the target 30normal to the surface thereof. The electron discharge device describedabove is substantially of conventional design and any suitable type ofstructure may be utilized with the exception of the target assemblywhich is a new and improved target electrode.

Referring now to FIG. 2, for a more detailed description of the target30. The target 30 is supported on the light-transmission faceplate 14 ofa material such as glass. The target 30 consists of electricalconductive coating 32 of a thickness of about 500 angstroms of tin oxideand transmissive to the input radiations. The resistance of material inthe layer 32 should be less than 200 ohms per square. The electricalconductive coating may be formed by spraying a solution of tin salt overthe heated support faceplate 14. It is also possible to provide aconductive coating of a material such as gold according to well knownand established techniques by evaporation onto the faceplate.

After the electrically conductive coating 32 has been provided on thefaceplate, the structure is placed in a vacuum of about 10' torr and alayer 34 of a suitable insulating material of low dielectric constant ofabout 1.36, such as cryolite (Na AlF is deposited on the conductivecoating 32. The insulating coating 34 has a thickness of about 100 to1500 angstroms. The substrate should be heated to a temperature of 100to 250 C. prior to the deposition of the insulator to prevent anyoutgassing of the support material during subsequent depositions, and toinsure good adhesion between the coatings 34 and 32. The deposition ratemust be rapid to prevent any increase in the dielectric constant. Atypical evaporation consists of about 80 milligrams of cryoliteevaporated at a distance of about eight inches. The speed of evaporationis about ten seconds. The layer 34 must be at least 100 angstroms thickin order to yield an insulating layer which is not too high incapacitance. Thicker layers may be used but they should not exceed 1500angstroms.

The layer 36 of asuitable photoconductive material including arsenic andselenium such as As Se is evaporated onto the layer 34. Other suitablematerials are CdSe mixed with As Se CdS mixed with As Se or As Se S.This layer 36 may be produced by evaporation in a container having anatmosphere of a suitable inert gas such as argon at a pressure of about1001b. The layer 36 should be of a thickness of about 0.2 to 0.3 micron.This provides a porous layer having a density of less than percent ofits normal bulk density, The proper thickness is normally determined bymonitoring transmission of light through the substrate in a well knownmanner to provide transmission of about 10 percent of the light. A

4 solid layer 37 is provided over the layer 36 by evaporating in avacuum of about l0- torr, one of the above materials and to a thicknesssuch that only about 1 percent of the light is transmitted. Thiscorresponds to a thickness of about 2 to 3 microns for the solid layer.This layer is at approximately its normal bulk density.

It is found that a target fabricated in the above manner provides a highsensitive vidicon type tube with short response time.

In FIG. 3, the fast decay properties of the target are illustrated. FIG.3 indicates that the signal decays to less than 25 percent of themaximum signal in three scans, each ,6 of a second. In the absence ofthe scanning beam, the target will retain the stored imaged for at least10 to 15 seconds. This makes the tube suitable for slow scan operation.

FIG. 4 illustrates the fast build up of the target. FIG. 5 illustratesthe sensitivity of the target and FIG. 6 compares the dark current ofthis invention illustrated by curve 51 with that of the prior art typeof device illustrated by curve 52.

While there have been shown and described what are presently consideredto be the preferred embodiments of the invention, modifications thereofwill readily occur to those skilled in the art. For example, thephotoconduct-ive layers may be reversed with the solid layer on theinsulator and the porous layer on the solid layer. It is not desired,therefore, that the invention be limited to the specific arrangementshown and described, and it is intended to cover in the appended claimsall such modifications as fall within the true spirit and scope of theinvention.

I claim as my invention:

1. A pickup tube comprising an evacuated envelope containing means todevelop a scanning beam, a radiation sensitive target assembly withinsaid envelope to be scanned by said electron beam, said target assemblycomprising an electrically conductive film transmissive to inputradiations, a layer of cryolite of a thickness of to 1500 angstroms, afirst photoconductive layer including arsenic and selenium deposited onsaid cryolite layer and having a density not greater than 10% of itsnormal bulk density and a second coating of photoconductive materialincluding arsenic and selenium deposited on said first photoconductivelayer, said second photoconductive layer having a substantially normalbulk density.

2. A pickup tube comprising an evacuated envelope containing means todevelop a scanning beam, a radiation sensitive targe assembly withinsaid envelope to be scanned by said electron beam, said target assemblycomprising an electrically conductive film transmissive to inputradiations, a layer of cryolite of a thickness of 100 to 1500 angstroms,a first photoconductive layer including arsenic and selenium depositedon said cryolite layer and having substantially normal bulk density anda second coating of photoconductive material including arsenic andselenium deposited on said first photoconductive layer, said secondphotoconductive layer having a density not greater than 10% of itsnormal bulk density.

References Cited UNITED STATES PATENTS 2,887,596 5/1959 Rijssel et al.2,967,254 l/196l Forgue. 3,069,551 12/1962 Haine 315-10 X 3,046,431 7/1962 Nicholson. 3,213,315 10/1965 Lempert. 3,268,764 8/1966 Simms.

ROBERT SEGAL, Primary Examiner.

